This invention relates to boiler furnaces having a bottom positioned grate on which wood bark or the like is burned, while slag producing fuel such as pulverized coal is simultaneously burned in suspension as it is projected from one or more burners located at the side of the boiler furnace and above the grate. When ash resulting from burning coal is subjected to high boiler temperatures it becomes fused and is carried by the gases toward the outlet thereof where it may adhere to furnace walls or other surfaces exposed to high gas temperatures.
Impingement of this fused ash upon a furnace wall results in its cooling and solidification that can cause the formation of heavy layers of dense slag on the furnace walls. Large accumulations of ash or slag eventually break loose and fall to the grate of the stoker to cause physical damage thereto and disturb the burning process that is being effected thereon.
As a result of the present energy crisis a growing number of pulp mills and the like are finding it economical to install bark-fired boilers. At the same time they are finding it advantageous to use pulverized coal as a primary fuel, but for reasons above outlined, the simultaneous use of a coal burner and a stoker has a potential for extreme stoker damage, ineffective combustion, and frequent forced outages that result in a low boiler efficiency.
The present invention is therefore directed towards providing a pulverized coal fired boiler with a subjacent movable grate that can operate simultaneously without the harmful effects caused by accumulations of heavy fused ash and slag falling onto the surface of the movable grate.